A category of musical instruments known as "keyboard percussion instruments" includes marimbas, vibraphones, xylophones and glockenspiels. The "keys" on a keyboard percussion instrument are known as "tone bars." A basic feature of a keyboard musical percussion instrument is a suspension system that holds the tone bars in place, while allowing them to ring freely. Various types of suspension systems have been developed.
In the earliest xylophones, the tone bars were suspended on strips of straw. For this reason, they were sometimes called a "stroefiddle" or "straw fiddle." Beginning in the early part of this century, the tone bars of keyboard musical percussion instruments were similarly suspended on strips of wool felt. In many cases, the bars of these early instruments were not drilled. Instead, they were held in place on a frame by crisscrossed straps or string. Although such straw or felt suspension systems are no longer used on marimbas, xylophones and vibraphones, in some glockenspiels with steel tone bars, felt strips are still used under the tone bars.
The earliest wooden keyboard percussion instruments manufactured in North America, i.e., the xylophone and the marimba, used a suspension system developed in Central America during the 19th century. This system employed horizontal holes drilled entirely through the width of the tone bar at the two average nodal points of the bar. (A nodal point, or non-vibrating point, on a percussive tone bar is typically located approximately one quarter bar length from each end of the bar.) After all of the tone bars are drilled for an entire keyboard, i.e., two holes per bar, a string or cord was passed through the holes. The entire keyboard was then suspended on a series of "stand-offs" or "bar posts" located between each tone bar. This drill-and-string suspension system is still used on professional-quality marimbas and xylophones.
In the drill-and-string system, twice the width of the tone bar is in contact with the suspension cords. Thus, in the case of a low-range marimba bar 3.5 inches wide, 7 inches of tone bar are in contact with the suspension cord. This extensive contact produces a tone-dampening effect. In addition, the density, grain, elasticity and growth patterns of wood inevitably vary, and the non-uniform properties of a wooden tone bar can produce non-uniform, unpredictable nodal paths. Any undesirable dampening resulting from contact with the suspension cord becomes more pronounced in wooden bars, because the cord is frequently in contact with live, i.e., vibrating, regions of the bar. Other disadvantages of a drill-and-string suspension system include cord breakage, audible vibration of the cord against the tone bar, and the considerable manufacturing expense associated with drilling two holes in each of as many as sixty-one bars (on a five-octave marimba) made of steel or very hard wood.
More recently, in many commercial musical keyboard percussion instruments, the tone bars are attached upon each of the pair of rails by a screw engaged loosely through a hole extending from the top surface, through the tone bar, into the respective support rail therebelow, the screws being located at each of the two nodes of the fundamental mode of vibration, i.e., at "Node A" and "Node B" of tone bar.
In a prior art commercially available glockenspiel, each tone bar is supported at four points by a pair of bumpers resting on each of a pair of support rails, with the tone bar secured by a pin extending from one rail into a vertical through hole in the tone bar. The pairs of bumpers and the pin are located at each of the two nodes of the fundamental mode of vibration, i.e., at "Node A" and "Node B" of tone bar.